Scenario: Go either to the Updates section or the Plugins page, select more than one plugin to update (or just select all), and try to update.
Result: The maintenance file will be added to the project root, some files will be downloaded, but not all, and nothing will seemingly complete. In addition, the upgrade progress will not be displayed, and as a grande finale, you'll end up with a white screen of death when you try to click on any links.